A 250GB platter will not come close to performing like the 320GB platter WD6400AAKS drives. For $75 it is THE drive to buy right now.

For most parts the WD6400AAKS will be a great boot drive. The Raptor is a great boot drive as well, it shines because of the fast seek times. It is throughput where it falls behind the bigger plattered drives out there. The difference between them isn't going to be huge though. If you think that the money difference is worth it, then go with the Raptor.

Have you seen the specs on the 250g drive?
If this isn't quick I don't know what is.